Hillary Clinton’s health – long the obsession of conspiracy theorists — emerged Sunday as a legitimate campaign issue after Clinton nearly swooned and stumbled at a Sept. 11 commemoration, underscoring the sense that that summer’s sure-thing candidate is flagging at a pivotal moment.

The scare was captured on cellphone video showing the wobbly Democratic candidate — who was diagnosed with pneumonia on Friday, according to a report by her doctor and later forwarded to reporters — being lifted into the vehicle by her aides after leaving a memorial service at the site of the 2001 World Trade Center attack.

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The incident comes after two weeks of tightening polls that have seen Trump close to within striking distance nationally and in some battleground states and capped off what was arguably the shakiest 48 hours of her general-election campaign that began when she called Trump supporters “deplorables,” a rare gaffe by a fastidiously stage-managed candidate.

The announcement late Sunday by Clinton’s personal physician, Lisa Bardack, that Clinton had become “overheated and dehydrated” after being placed on antibiotics last week came after a day of frenzied speculation about the candidate’s health and its impact on a tightening race.

“It’s not the end of the world — I think she’s OK — but it ain’t good,” one person close to the Clintons said after the former secretary of state emerged from daughter Chelsea’s Manhattan apartment after becoming “overheated” at Ground Zero. “It adds a new, unwelcome element.”

Longtime Clinton adviser James Carville said, ‘The doctors have all said she’s healthy,” but added: “It’s going to be something that people aren’t going to be able to stop talking about, so we’ll see” what happens next.

Even as reporters waited for more details about the health scare, supporters — for the first time — were quietly pressuring a candidate who has been reluctant to share details about her health and personal life to become more transparent to avoid a drip-drip of doubt that could help an even less forthcoming Trump.

The announcement that the candidate is suffering from a lung ailment — which came after her staff stonewalled reporters and pronounced her fully recovered — isn’t likely to instantly quell questions about her long-term fitness.

"The Clintons have this notion that they only go as far as they need to" in terms of disclosure, said an ally close to the current Democratic nominee. "They can't afford to approach this that way."

In a wild-card campaign shaped by branding, spin and bombast, Clinton’s health scare is a rare event rooted in flesh-and-blood truth — she’s either healthy or she’s not — and on Sunday that question took on an unanticipated urgency.

Clinton’s campaign strategy is driven by an intent to demonstrate Trump’s unfitness to serve based on temperament and intellect. The GOP nominee now has a powerful, visceral counter-argument: that she is physically unfit to serve, reinforced by real video that raises new concerns about her health.

Presidential politics is, by design, a brutal vetting process, and the physical capacity of anyone seeking the office is the ultimate gateway question. But in Trump’s hands it has become positively gladiatorial, thanks to his chest-beating emphasis on his own physical prowess, size, and stamina.

Partisans have long raised health issues in campaigns — Ronald Reagan’s opponents repeatedly questioned his mental acuity, and Bill Clinton’s supporters were delighted by a January 1992 video of George H.W. Bush vomiting during a dinner with the prime minister of Japan. Dwight Eisenhower's 1955 heart attack and subsequent health issues were a factor in his 1956 reelection race.

But Trump has personally ridiculed Clinton’s health with a bluntness not seen in recent times, suggesting she’s been hobbled by a serious 2012 head injury and mocking her repeated allergy-induced coughing fits. “Mainstream media never covered Hillary’s massive ‘hacking’ or coughing attack, yet it is #1 trending. What’s up?” he tweeted after a coughing fit interrupted a Clinton speech on Sept. 6.

“Donald Trump is hardly a physical specimen. He’s a 70-year-old fat guy who doesn’t exercise — let him run a hard mile on a treadmill,” said Stuart Stevens, Mitt Romney’s top strategist in 2012 and a frequent critic of Trump. “What does Hillary Clinton’s health have to do with making Donald Trump more attractive or vice versa? This all seems to be kind of like mascot stealing. People get all excited about it and they think it’s going to help their team win, but it doesn’t seem to effect what happens on the field.”

People close to Clinton dismissed the incident as an outlier.

Veteran Clinton fundraiser Alan Patricof, was stunned when a POLITICO reporter asked about her overheating episode on Sunday and said the Democratic nominee had never looked better than when he saw her last month in the Hamptons. "I saw her the other week at the Paul McCartney event … I never saw her look better or happier,” he said. “She was having a hell of a good time. She was at her peak, dancing. The last time I saw her dancing was a long time ago."

“I'm not worried at all,” Former Pennsylvania Gov. Ed Rendell said. “As long as she performs well and looks strong in the debates, that's all that matters.”

Despite her bouts with seasonal allergies, Clinton has appeared energetic at her public events — although one person in her orbit said that her decision to spend August fund-raising, off the trail, also served the purpose of “resting up” for the final fall push.

And several other party leaders and Clinton backers, who didn’t speak on the record to avoid antagonizing Clinton and her Brooklyn-based campaign, told POLITICO she needed to offer a fuller accounting of her health – especially after a transcript of Clinton’s FBI interview regarding her email server showed that she “could not recall every briefing she received” as a result of “a concussion” and “blood clot” around New Year’s 2012.

“There will have to be some kind of accounting for this,” said one former Clinton adviser – on a day when Clinton’s staff kept her traveling press pool in the dark about her condition for most of the day. “Whether she decides to do that, or just take the hits, that’s her call.”

By Sunday afternoon, the 68-year-old Clinton was already facing calls to release her full medical records — something that her 70-year-old, corpulent, junk-food-gobbling opponent, who has proudly posed with a KFC chicken bucket, has thus far refused to do. Trump’s own physician has admitted that the hyperbolic clean-bill-of-health he dashed off for the candidate was written to please his famous patient.

(A Trump rapid response staffer emailed POLITICO to complain that it was “a joke” to suggest his boss was overweight or ate poorly.)

Still, the incident has ratcheted up pressure on both candidates to provide a serious accounting of their health. “She ought to be come forward and talk about it,” said Pat Buchanan, who did not bring up then-President Bush’s health scare when he unsuccessfully challenged him for the GOP nomination in 1992.

Now he’s calling for the release of “the full medical records” on both candidates.

“Having worked in a White House for a full first term, I can tell you that is a very physically and emotionally demanding job,” said Matt Schlapp, who served as George W. Bush’s political director and helped handle the fallout of an embarrassing incident where the commander-in-chief nearly choked on a pretzel while watching TV with his dog.

“One of the good things about the way we run for president is that it also is very physically and emotionally exhausting, and in that way it serves as a good dry run,” he added. “We have two candidates who are senior citizens and I think it is good and right that the American public demands a full accounting of their health.”

Sunday began solemnly and without incident. Clinton, along with Trump, attended the 15th anniversary commemoration of the attacks in New York, Washington and Pennsylvania, appearing to be cheerful and lucid, as she walked into the site wearing dark glasses. A campaign official said she intended to stay for the entire event, until about 10:30, but a Clinton spokesman said the candidate decided to leave at about 9:30 when she began feeling “overheated.”

Several other Democrats, including Rep. Joe Crowley (D-N.Y.) said the site was unshaded and hot — and many of those in attendance had sweated through their clothes — even though the temperature was a moderate 82 degrees with mild humidity.

Clinton walked, with the assistance of an aide, screened by her protective detail, to an area adjacent to the ceremony where she stood unsteadily until a van pulled up to take away; Cellphone video shows her knees buckling as she entered the vehicle and eyewitnesses standing nearby told reporters she lost one of her shoes under the van.

The small pool of reporters covering the event were restricted to a press pen for 90 minutes after the incident, and given no information until Clinton staffer Nick Merrill issued a statement saying the former secretary of state had been driven to her daughter’s apartment, was cooling off, and “felt much better.”

About an hour later, Clinton, wearing shades and toting a heavy handbag, walked slowly — and conspicuously un-staffed — out onto the sidewalk, waved to the crowd and gathered media, stopping to pose for a snapshot with a young girl, before climbing into an SUV for the drive back to her house in the northern New York suburbs. “I'm feeling great, it's a beautiful day in New York," she said in a voice loud enough to be heard by the TV and radio microphones.

By mid-afternoon, Clinton’s press pool had been gathered at a hotel near her house in Chappaqua, and waited two hours for word from the campaign.

“Secretary Clinton has been experiencing a cough related to allergies,” wrote Bardack. “On Friday, during follow up evaluation of her prolonged cough, she was diagnosed with pneumonia. She was put on antibiotics, and advised to rest and modify her schedule. While at this morning's event, she became overheated and dehydrated. I have just examined her and she is now re-hydrated and recovering nicely."

The question now, political not medical, is how Trump will handle the situation.

Trump, whose recent gains in the polls have coincided with a more restrained, stage-managed style, brushed off reporters’ requests to comment on Clinton’s problems.

Privately, his supporters cast the developments as a positive for the campaign, but worried that Trump and his supporters would overshoot the mark – reinforcing her claims that he’s “temperamentally unfit” -- as he has done so many times in the past.

Twitter overflowed with Trump supporters celebrating her misfortune, with some backers claiming she was “old and sick,” while others spread the false rumor that Clinton had “stroked out.” One top Iowa GOP official tweeted, and then deleted, “Can’t handle the heat, get out of the race. #HillaryforHospice.”

He quickly apologized, reflecting Republicans’ calculation that the incident could help Trump so long as they are not seen as gloating. “I made a mistake in hastily tweeting from my personal account a poorly-constructed joke that was in no way funny nor amusing.”

One Republican lobbyist who raises money for Trump offered the campaign similar advice: “Trump should not say or tweet a thing about Hillary’s health, unless it’s to say, ‘I hope she feels better.’ He needs to be very careful and sensitive, and not reinforce the worse things people think about him, which is that he’s an asshole. And at a time like this, he should let his top three or four surrogates argue that voters deserve to see her full health records.”

A person close to Trump hoped – and doubted – he could toe that line. “You're asking for a lot of nuance from a guy who doesn't really do nuance well,” the person told POLITICO.